1. Field of the Invention
This invention relates to a new method for the production of 3,3',4,4'-tetraaminobiphenyl (TAB).
High molecular weight polybenzimidazoles are polymers of high thermal stability and excellent resistance to oxidative or hydrolytic degradation, which can be formed into shaped articles such as fibers, films, and molded articles having very desirable physical properties. As taught by the published literature, these polymers may be prepared, for example, by melt polymerizing an aromatic tetraamine and an aromatic or heterocyclic dicarboxylic acid, ester or anhydride in a one or two stage process; see, for example H. Vogel and C. S. Marvel, Journal of Polymer Science, Vol. L, pages 511-539 (1961); and U.S. Pat. Nos. 26,065; 3,174,946; 3,509,108; 3,551,389; 4,312,976; 3,433,772; and 3,655,632. The most widely used aromatic tetraamine for the production of these polybenzimidazoles is 3,3',4,4'-tetraaminobiphenyl (TAB), while the most commercially significant polybenzimidazole is poly-2,2'-(m-phenylene)-5,5'-bibenzimidazole, which is prepared by polymerizing TAB with a difunctional isophthaloyl compound, such as diphenyl isophthalate or isophthalic acid.
The method for producing TAB hitherto considered to be the most desirable utilizes 3,3'-dichlorobenzidine (DCB) as a starting material. In this method, DCB is subjected to direct ammonolysis with a cuprous chloride catalyst. Variations of this method are shown, for example, in U.S. Pat. Nos. 3,865,876 and 3,943,175.
In another method for producing TAB which has engendered substantial interest, the starting material is benzidine which is acetylated, e.g., with acetic anhydride, to form N,N-diacetylbenzidine. The latter compound is then nitrated with concentrated nitric acid to form 3,3'-dinitro-N,N-diacetylbenzidine which is base hydrolyzed to form 3,3'-dinitrobenzidine. This is then reduced by any of various means to form TAB. An example of this method is disclosed by H. Vogel and C.S. Marvel, J. Poly. Sci., Part Al, 1531 (1963).
Both of the foregoing methods have the disadvantages that they utilize starting materials, viz. DCB and benzidine, which are known carcinogens. This makes such methods relatively expensive and difficult to carry out. Thus, any method for producing TAB utilizing a raw material which is both safer and easier to handle, and is substantially less expensive than DCB and benzidine, would be very desirable.